Without the cross, the two triangles are plainly the same gray.
Lighter or darker?
Benary cross
Which small triangle is darker: the one in the corner of the cross, or the one on the cross?
Neither: they are the same gray, though the one in the corner looks a little darker.
Why? One triangle seems to belong to the cross and the other to the background, and your brain judges each against the thing it belongs to.
Picture: IdS, Public domain